Chapter 16: Security
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Chapter 15: Security
●
The Security Problem
● Authentication
● Program Threats
●
System Threats
●
Securing Systems
●
Intrusion Detection
●
Encryption
●
Windows NT
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PROTECTION & SECURITY
●
Are implemented to prevent interference with the use of files.
● Threats:- Protection & Security
● Protection: is strictly internal problem. How do we provide
controlled access to the programs and data stored in computer
system.
●
Security: on the other hand, deals with the external threats.
●
includes firewalls, encryption techniques.
●
We say our system is secure if its resources are used and
accessed as intended under all circumstances.
●
Computer resources must be guarded against unauthorized
access, malicious destruction or alteration.
●
Total security can never be achieved.
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The Security Problem
●
Security must consider external environment of the system, and
protect it from:
●
unauthorized access.
●
malicious modification or destruction
●
Data unavailability.
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Security violation can be categorized as intentional or accidental.
●
Easier to protect against accidental than intentional misuse.
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Some common Terms
1.
Threat: is the potential to security violation.
2. Attack: attempt to break security.
3.
Hacker:
⚫
A person with a strong interest in computers who enjoys
learning and experimenting with them.
⚫
Cracker/Intruder:
Someone who breaks into computers.
Crackers should not be confused with hackers.
The term cracker is usually connected to computer criminals.
Some of their crimes include ID theft and session hijacking
and intrusion in unauthorized areas.
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Some security violations:
●
Breach of confidentiality: Unauthorized reading of data. Theft of
information
● Breach of integrity: Unauthorized modification of data.
●
Breach of availability: Unauthorized destruction of data/ website
defacement: Awebsite defacementis an attack on a website
that changes the visual appearance of the site or a webpage.
These are typically the work ofsystem crackers, who break into
aweb serverand replace the hostedwebsitewith one of their
own. SQL injections.
●
Theft of service: unauthorized use of resources.
●
Denial of service: Send invalid data to applications or network
services, which cause abnormal termination or behavior of the
applications or services.
⚫
Flood a computer or the entire network with traffic until a
shutdown occurs because of the overload.
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●
Attackers use several standard methods in their attempts to
breach security:
Masquerading: pretend to be someone else.
Replay attack: repeat of valid data transmission
Man in the middle attack: attacker sits in the data flow of
communication. Masquerading as the sender to the receiver and
vice versa.
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To protect our system, we must take security measures at 4 levels:
● Physical (site containing computer system must be physically
secured).
●
Human(using authorization/authentication mechanisms)
●
Operating system(using firewalls)
●
Network(using encryption-decryption techniques)( RSA,
DES(DATA ENCRYPTION STANDARD,AES(ADVANCE
ENCRYPTION STANDARD,MESSAGE DIGEST 5)
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Authentication
●
User identity most often established through passwords.
● Passwords must be kept secret.
●
Frequent change of passwords.
●
Use of “non-guessable” passwords.
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Log all invalid access attempts.
●
Passwords may also either be encrypted or allowed to be used
only once.
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ACCESS MATRIX
● Access Matrix is used to specify
the resources a process may access.
●
Protection Domain-Domain can be
user, process, procedure
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Domain Structure
● Access-right = <object-name, rights-set>
where rights-set is a subset of all valid
operations that can be performed on the
object
●
Domain = set of access-rights
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Access Matrix
● Used for the implementation of
Protection Model.
●
Rows represent domains
●
Columns represent objects
●
Access(i, j) is the set of operations
that a process executing in Domain
i
can invoke on Object
j
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Access Matrix
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Types of Threats
Program Threats
● When any prog.created by a user is used by another user then
misuse of that prog.may occur.Examples:-Trojan Horses,Trap doors,
stack buffer overflow.
●
Malware(malicious software): software that is specifically
designed to disrupt, damage, or gain unauthorized access to a
computer system Types of malware can include
computerviruses,worms,Trojan horses and spyware.
●
A back door is a means of access to a computer program that
bypasses security mechanisms. A programmer may sometimes
install a back door so that the program can be accessed for
troubleshooting or other purposes. However, attackers often use
back doors that they detect or install themselves, as part of
anexploit. In some cases, awormis designed to take advantage
of a back door created by an earlier attack.
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Trojan horse
●
ATrojan horse, orTrojan, is software that appears to perform a
desirable function for the user, but (perhaps in addition to the
expected function) steals information or harms the system.
●
It is a program that sit ideally and transmits all user information to
the attacker.
●
It is attached to your browser and when user fills his username or
password for any website, it records everything and passes to
attacker.
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Spyware
●
Spywareis a type ofmalwarethat can be installed oncomputers,
and which collects small pieces of information about users
without their knowledge.
●
The presence of spyware is typically hidden from the user, and
can be difficult to detect.
●
Sometimes, however, spywares such askey loggers are installed
by the owner of a shared, corporate, orpublic computer on
purpose in order to secretly monitor other users.
●
While the termspywaresuggests software that secretly monitors
the user's computing. Spyware programs can collect various
types of personal information, such asInternet surfinghabits and
sites that have been visited, but can also interfere with user
control of the computer in other ways, such as installing
additional software and redirectingWeb browseractivity.
●
Spyware is known to change computer settings, resulting in slow
connection speeds, different home pages, and/or loss
ofInternetconnection or functionality of other programs.
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● Incomputer security, acovert channelis a type of
computer security attack that creates a capability to
transfer information objects between processes that are
not supposed to be allowed to communicate by the
computer security policy.
● Alogic bombis a piece ofcodeintentionally inserted into
asoftware system that will set off a malicious function
when specified conditions are met. For example, a
programmer may hide a piece of code that starts
deletingfiles(such as asalarydatabase trigger).
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System Threats-OS files/resources are
misused
● VIRUS: Vital Information Resource Under Seizeor Very
Important Resource Under Seize ..both are correct.
● Small progs. Written to alter the way a computer operates.
● designed to “infect” other programs.
● Executes without user permission.
● Can replicate itself.
● Once a virus reaches a target machine, a program known
as virus dropper inserts virus onto system..
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Worms
● Small programs that do -Self propagating or self
duplicating.
● Contains malicious code that cause major damage to OS
files.
● If a device gets infected, it sends the copies of itself onto
the network to other devices.
● Sometimes more dangerous, it might go into your email,
find your contacts, sends copies of itself to all the contacts.
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Threat Monitoring
●
Check for suspicious patterns of activity – i.e., several incorrect
password attempts may signal password guessing.
● Audit log – records the time, user, and type of all accesses to an
object; useful for recovery from a violation and developing better
security measures.
●
Scan the system periodically for security holes; done when the
computer is relatively unused.
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Threat Monitoring (Cont.)
●
Check for:
● Short or easy-to-guess passwords
●
Unauthorized set-uid programs
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Unauthorized programs in system directories
●
Unexpected long-running processes
●
Improper directory protections
●
Improper protections on system data files
●
Changes to system programs: monitor checksum values
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FireWall
●
Afirewallis a device or set of devices designed to permit or deny
network transmissions based upon a set of rules and is frequently
used to protect networks from unauthorized access while
permitting legitimate communications to pass.
●
A firewall is placed between trusted and untrusted hosts.
●
The firewall limits network access between these two security
domains.
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Encryption
●
Encrypt clear text into cipher text.
● Substitution and transposition ciphers
● Properties of good encryption technique:
●
Relatively simple for authorized users to encrypt and decrypt
data.
●
Encryption scheme depends not on the secrecy of the
algorithm but on a parameter of the algorithm called the
encryption key.
●
Extremely difficult for an intruder to determine the encryption
key.
●
Data Encryption Standard substitutes characters and rearranges
their order on the basis of an encryption key provided to
authorized users via a secure mechanism. Scheme only as
secure as the mechanism.
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Encryption (Cont.)
●
Public-key cryptographyrefers to acryptographicsystem
requiring two separate keys, one to lock or encrypt theplaintext,
and one to unlock or decrypt thecyphertext.
●
Neither key will do both functions.
●
One of these keys is published or public and the other is kept
private.
●
Public key cryptography was invented in 1976 by Whitfield Diffie
and Martin Hellman. For this reason, it is sometime calledDiffie-
Hellman encryption.
●
It is also calledasymmetric encryptionbecause it uses two keys
instead of one key (symmetric encryption).
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Private key encryption
● Symmetric encryption(also calledprivate-key
encryptionorsecret-key encryption) involves using the
same key for encryption and decryption.
● Encryption involves applying an operation (an algorithm) to
the data to be encrypted using the private key to make
them unintelligible.
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15.07
End of Chapter 15
Chapter 12: Secondary-Storage
Structures
Overview of Mass Storage Structure
●
Magnetic disks provide bulk of secondary storage of modern computers
● Drives rotate at 60 to 200 times per second
● Transfer rate is rate at which data flow between drive and computer
●
Positioning time (random-access time) is time to move disk arm to
desired cylinder (seek time) and time for desired sector to rotate
under the disk head (rotational latency)
●
Head crash results from disk head making contact with the disk
surface
● Disks can be removable
● Drive attached to computer via I/O bus
Moving-head Disk Machanism
Disk Scheduling
●
The operating system is responsible for using hardware efficiently
— for the disk drives, this means having a fast access time and
disk bandwidth.
●
Access time has two major components
●
Seek time is the time for the disk are to move the heads to the
cylinder containing the desired sector.
●
Rotational latency is the additional time waiting for the disk to
rotate the desired sector to the disk head.
●
Minimize seek time
●
Seek time ≈ seek distance
Disk Scheduling (Cont.)
●
Several algorithms exist to schedule the servicing of disk I/O
requests.
● We illustrate them with a request queue (0-199).
98, 183, 37, 122, 14, 124, 65, 67
Head pointer 53
FCFS
Illustration shows total head movement of 640
cylinders.
SSTF
●
Selects the request with the minimum seek time from the current
head position.
● SSTF scheduling is a form of SJF scheduling; may cause
starvation of some requests.
●
Illustration shows total head movement of 236 cylinders.
SSTF (Cont.)
SCAN
●
The disk arm starts at one end of the disk, and moves toward the
other end, servicing requests until it gets to the other end of the
disk, where the head movement is reversed and servicing
continues.
●
Sometimes called the elevator algorithm.
●
Illustration shows total head movement of 208 cylinders.
SCAN (Cont.)
C-SCAN
●
Provides a more uniform wait time than SCAN.
● The head moves from one end of the disk to the other. servicing
requests as it goes. When it reaches the other end, however, it
immediately returns to the beginning of the disk, without servicing
any requests on the return trip.
●
Treats the cylinders as a circular list that wraps around from the
last cylinder to the first one.
C-SCAN (Cont.)
C-LOOK
●
Version of C-SCAN
● Arm only goes as far as the last request in each direction, then
reverses direction immediately, without first going all the way to
the end of the disk.
C-LOOK (Cont.)
Selecting a Disk-Scheduling Algorithm
●
SSTF is common and has a natural appeal
● SCAN and C-SCAN perform better for systems that place a heavy
load on the disk.
●
Performance depends on the number and types of requests.
●
Requests for disk service can be influenced by the file-allocation
method.
●
The disk-scheduling algorithm should be written as a separate
module of the operating system, allowing it to be replaced with a
different algorithm if necessary.
●
Either SSTF or LOOK is a reasonable choice for the default
algorithm.
Disk Management
●
Low-level formatting, or physical formatting — Dividing a disk into
sectors that the disk controller can read and write.
● To use a disk to hold files, the operating system still needs to
record its own data structures on the disk.
●
Partition the disk into one or more groups of cylinders.
●
Logical formatting or “making a file system”.
●
Boot block initializes system.
●
The bootstrap is stored in ROM.
●
Bootstrap loader program.
●
Methods such as sector sparing used to handle bad blocks.
Types of Devices
●
• Despite the multitude of devices that appear
(and
● disappear) in the marketplace and the swift rate of
●
change in device technology, the Device Manager
●
must manage every peripheral device of the
system.
●
• It must maintain a delicate balance of supply
and
●
demand – balancing the system’s finite supply of
●
devices with users’ almost infinite demand for
them.
●
Device management involves four basic functions:
● – Monitoring the status of each device, such as storage drives,
printers, and other peripheral devices;
●
– Enforcing preset policies to determine which process will get a
device and for how long;
●
– Allocating the devices;
●
– Deallocating them at two levels:
●
• At the process (or task) level when an I/O command has been
executed and the device is temporarily released;
●
• At the job level when the job is finished and the device is
permanently released.
Types of Devices (cont’d)
●
• The system’s peripheral devices generally fall
into
● one of three categories:
●
– Dedicated
●
– Shared
●
– Virtual
●
• The differences are a function of the
characteristics of the devices, as well as how
they’re managed by the Device Manager.
●
Dedicated Devices
● – Are assigned to only one job at a time.
● – They serve that job for the entire time the job is activen or until it
releases them.
●
– Some devices demand this kind of allocation scheme, because
it would be awkward to let several users share them.
●
• Example: tape drives, printers, and plotters
●
– Disadvantages
●
• They must be allocated to a single user for the duration of a
job’s execution, which can be quite inefficient, even though the
device is not used 100% of the time.
●
Shared Devices
● – Can be assigned to several processes.
●
– For example – a disk (DASD) can be shared by
several processes at the same time by interleaving
their requests;
●
• This interleaving must be carefully controlled by
the Device Manager– All conflicts must be
resolved based on predetermined policies.
● Virtual Devices
● – A combination of the first two types;
● – They’re dedicated devices that have been transformed
into shared devices.
● • Example: printer
● – Converted into a shareable device through a spooling
program that reroutes all print requests to a disk.
● – Only when all of a job’s output is complete, and the
printer is ready to print out the entire document, is the output
sent to the printer for printing.
● – Because disks are shareable devices, this technique can
convert one printer into several virtual printers,
thus improving both its performance and use.
Types of Devices (cont'd.)
● • Regardless of the specific attributes of the device,the
most important differences among them are speed and
degree of sharability.
● • Storage media are divided into two groups:
● – Sequential Access Media
● • Store records sequentially, one after the other.
● – Direct Access Storage Devices (DASD)
● • Can store either sequential or direct access files.
● • There are vast differences in their speed and sharability.